1. Field of the Invention
This invention relates to improvements in the cleaning of solid materials or molded articles by treating them in a washing or cleaning bath under simultaneous action of ultrasound on the bath liquid and/or the material to be cleaned.
2. Discussion of Related Art
The elimination especially of pigmented and/or fatty soils from the surface of solid workpieces or molded articles in laundering and cleaning baths with simultaneous use of ultrasound for easier soil removal today is a definite part of the relevant process technology, wherein the total area, from an occasional cleaning process, for example, in the cleaning of ancient coins, to an independent process step in the continuous industrial scale manufacture, for example, of motor vehicles, is covered. A considerable body of literature exists concerning the mathematical laws, the units and the application areas of ultrasound cleaning technology, wherein the following may be mentioned as fairly recent publications: R. Sievers, Facilities and Application Areas of Ultrasonic Cleaning Technology, in: Metall, Internationale Zeitschrift fur Technik und Wirtschaft, Vol. 35, pp. 763 ff, 1981; and Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Wiley-Interscience (1983), Vol. 23, pp. 462-479, especially 463-465.
Thus, the most important process elements even today include the following conditions; i.e., a frequency range of about 20-60 kHz, the use of chiefly piezoceramic and magnetostrictive oscillators, power densities in the sound-struck area in the range from about 5-15 W/l with some considerable deviations, mainly higher, and the use of aqueous, and especially aqueous-surfactant baths and/or organic solvents as the wash liquid. Chlorinated or fluorinated hydrocarbons (CHC or FHC) are mainly used as the organic solvents.
The facilitation of the cleaning effect is essentially based on cavitation and the extreme pressure fluctuations produced in this way in the immediate vicinity of the soiled solid surface. It is known that cavitation in aqueous cleaning solutions is harder and more effective than in organic solvent systems. Because of the extremely different types of soils, however, according to the existing technical possibilities, either aqueous and/or organic wash baths are used, depending on the soil. Organic solvents are especially suitable for degreasing, but not infrequently leave behind the degreased, pigment-like residues as a dry, dusty coating on the surface to be cleaned. Aqueous, especially alkaline, and possibly surfactant-containing wash baths are not very suitable for removal of fatty soils, but instead are suitable for removing pigment-like soils, or, at best, for removing readily emulsificable fatty coatings or residues on the solid surface.
It is true for both the aqueous and the organic washing and cleaning systems that the liquid phase rapidly becomes enriched with the washed-off soil in the case of continuous use, rapidly reaching a soil concentration which once again results in soil residues depositing on the surfaces to be cleaned, and thus inadequate cleaning performance. Especially in a continuous process, the constant replacement of the washing bath liquid phase by fresh bath liquid is necessary. This can be done batchwise or continuously in the units known to date. In general, regeneration of the soil-loaded liquid phase is provided. This is especially true for organic wash liquors, which are generally purified by distillation and returned to the cleaning process. However, the regeneration of aqueous wash bath liquids often requires such a considerable expense that instead the used bath liquid is discarded and replaced by freshly made up aqueous bath liquid.
The teaching of this invention proceeds from the object of substantially improving in many ways the cleaning of molded articles, and especially the cleaning of the surfaces of such molded articles from pigmented and/or fatty soils in suitable washing and cleaning baths with the simultaneous use of ultrasound. Thus, for example, the invention also seeks to enable the batchwise or continuous regeneration of the bath liquids, and precisely during the use of these baths, without distillation of the bath liquid being required. The advantages of the new development thus apply equally to organic bath liquids and to aqueous baths, wherein the invention in a particularly important embodiment has also taken on the task of partly or preferably completely avoiding organic wash baths, and especially in such instances where previously the use of chlorinated hydrocarbons and/or fluorinated hydrocarbons was considered indispensable. An additional object of the invention is that of cleaning effectively with aqueous bath liquids under relatively mild working conditions especially in terms of the pH value and the process temperature, wherein even the most difficult cleaning task should be possible in aqueous washing and cleaning baths using ultrasound in the neutral pH range and, at most, at moderately elevated temperatures. Finally, in a particular embodiment, the invention permits the removal of the soil from a bath liquid in such a way that it is possible essentially to operate with stationary baths by batchwise or continuous throughput of molded articles to be cleaned, without exhaustion of the cleaning capacity of the wash baths used taking place. Further specific objects of the invention will become apparent from the description of the invention which follows.